backend: only check bytes_done for engines without FIO_NOIO

backend: only check bytes_done for engines without FIO_NOIO

This basically reverts the functional part of commit 2557f7e6.
Instead of having to fiddle with the io_u manually, have
the backend check whether we expect data to have been transerred
or not.

Signed-off-by: Jens Axboe <axboe@kernel.dk>